Large-scale geometry and temporal variability of the Martian external magnetic field

Abstract:

The martian magnetic field is unique among the terrestrial planets, as it results

from the interaction of fields caused by crustal remnant magnetization and a planetary

ionosphere with the solar wind and the interplanetary magnetic field. Internal fields of

crustal origin have been subject to extensive studies, whereas the focus of our work deals

with average spatial structure and time variability in the martian external magnetic field.

We use the Mars Global Surveyor (MGS) vector magnetic field data to investigate the

large-scale geometry and magnitude of such external fields. We analyze the day-time and

night-time magnetic signature for the duration of the MGS mission in mapping orbit

(2000-2006). We use along-track vector field measurements to estimate the day-time

and night-time external fields after the subtraction of predicted crustal magnetic fields

at spacecraft altitudes. We also examine day/night differences (i.e., the daily variation)

in external fields; these are independent of crustal fields. Because the external fields are

modified by the crustal fields, we investigate their structure as a function of latitude in

the local time frame and as a function of both latitude and longitude in the body-fixed

frame. In the body-fixed-frame B_Θ is generally dominant in magnitude with a day/night

variation described to first order by a zonal degree-2 spherical harmonic structure.

B_r is strongly correlated with the crustal magnetic field. B_Φ shows variable spatial

behaviour during both night and day. Seasonal variations are observed as stronger

average magnetic fields in the hemisphere pointing towards the sun. Additional shorter

time scale variations in the global external field structure are observed.